The polymerization of alpha-olefins and mixtures thereof at low pressures is known to be promoted by coordination catalysts prepared from mixtures of compounds of (1) transition elements, and (b) organometallic compounds of elements of Groups IA to IIIA of the Periodic Table. Such polymerizations can be carried out in suspension, in solution, in the gas phase, and the like.
Because of favorable process economics, especially with low molecular weight olefins such as ethylene and propylene, it is frequently desirable to carry out olefin polymerization or copolymerization reactions in an inert diluent at a temperature at which the resulting polymer or copolymer does not go into solution; and where the polymer product is recovered without removing the polymerization catalyst. Thus, elaborate steps to remove catalyst from the polymer product are avoided. In order for this more economical method of polymer manufacture to be practical, the polymerization catalyst employed must be capable of producing polymer in high productivities in order to maintain the residual catalyst level in the final polymer at a very low level. Thus, the activity of an olefin polymerization catalyst is one important factor in the continuous search for catalysts useful for the polymerization of alpha-olefins. It is also desirable that the process used in forming such catalysts allow for ease of preparation and ready control over the properties of the final catalyst formed.
Another important aspect of a polymerization catalyst and the polymerization process employing same is the properties of the polymer particles produced. It is desirable to produce polymer particles which are characterized by strength, uniformity of size, and a relatively low level of fine particulate matter. Although polymer fluff having relatively high percentages of polymer fines can be handled with plant modifications, the production of polymers in high productivity with low level of fines content is highly desirable so as to avoid the need for such plant modifications. Especially desirable are polymer particles having a high bulk density and relatively uniform shape as well as size.
In an attempt to control catalyst fines and thus polymer fines in the past, various techniques have been applied. One example involves spray drying to obtain catalyst precursors of particular dimensions. Another technique has involved the prepolymerization of particulate catalyst. Still another technique has involved the employment of only catalyst particles having certain dimensions as separated by sieving.
One object of the present invention is to provide a method for providing a desirably shaped magnesium-containing catalyst precursor.
Another object of the present invention is to provide desirably shaped magnesium-containing olefin polymerization catalysts.
Still another object of the present invention is to provide a process for the polymerization of alpha-olefins which gives both desirable yields and polymer particles of a desirable size.
Still another object of the present invention is to provide olefin polymerization catalysts which have good activity and stereospecificity as well as desirable particle shape.
These and other aspects, objects, and advantages of the invention will become apparent to those skilled in the art from the following disclosure.